ovsdb-server: Don't drop all connections on read/write status change.

The commit [1] force drops all connections when the db read/write status changes.
Prior to the commit [1], when there was read/write status change, the existing
jsonrpc sessions with 'db_change_aware' set to true, were not updated with the
changed 'read_only' value. If the db status was changed to 'standby', the existing
clients could still write to the db.

In the case of pacemaker OVN HA, OVN OCF script 'start' action starts the
ovsdb-servers in read-only state and later, it sets to read-write in the
'promote' action. We have observed that if some ovn-controllers connect to
the SB ovsdb-server (in read-only state) just before the 'promote' action,
the connection is not reset all the times and these ovn-controllers remain connected
to the SB ovsdb-server in read-only state all the time. Even though
the commit [1] calls 'ovsdb_jsonrpc_server_reconnect()' with 'forced' flag
set to true when the db read/write status changes, somehow the FSM misses resetting
the connections of these ovn-controllers.

I think this needs to be addressed in the FSM. This patch doesn't address
this FSM issue. Instead it changes the behavior of 'ovsdb_jsonrpc_server_set_read_only()'
by setting the 'read_only' flag of all the jsonrpc sessions instead of forcefully
resetting the connection.

I think there is no need to reset the connection. In large scale production
deployements with OVN, this results in unnecessary waste of CPU cycles as ovn-controllers
will have to connect twice - once during 'start' action and again during 'promote'.

[1] - 2a9679e("ovsdb-server: drop all connections on read/write status change")

Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

ovsdb/jsonrpc-server.c

@@ -80,6 +80,8 @@ static void ovsdb_jsonrpc_session_unlock_all(struct ovsdb_jsonrpc_session *);
 static void ovsdb_jsonrpc_session_unlock__(struct ovsdb_lock_waiter *);
 static void ovsdb_jsonrpc_session_send(struct ovsdb_jsonrpc_session *,
                                        struct jsonrpc_msg *);
+static void ovsdb_jsonrpc_session_set_readonly_all(
+    struct ovsdb_jsonrpc_remote *remote, bool read_only);
 
 /* Triggers. */
 static void ovsdb_jsonrpc_trigger_create(struct ovsdb_jsonrpc_session *,
@@ -365,10 +367,13 @@ ovsdb_jsonrpc_server_set_read_only(struct ovsdb_jsonrpc_server *svr,
 {
     if (svr->read_only != read_only) {
         svr->read_only = read_only;
-        ovsdb_jsonrpc_server_reconnect(svr, true,
-                                       xstrdup(read_only
-                                               ? "making server read-only"
-                                               : "making server read/write"));
+
+        struct shash_node *node;
+        SHASH_FOR_EACH (node, &svr->remotes) {
+            struct ovsdb_jsonrpc_remote *remote = node->data;
+
+            ovsdb_jsonrpc_session_set_readonly_all(remote, read_only);
+        }
     }
 }
 
@@ -670,6 +675,17 @@ ovsdb_jsonrpc_session_reconnect_all(struct ovsdb_jsonrpc_remote *remote,
     }
 }
 
+static void
+ovsdb_jsonrpc_session_set_readonly_all(struct ovsdb_jsonrpc_remote *remote,
+                                       bool read_only)
+{
+    struct ovsdb_jsonrpc_session *s;
+
+    LIST_FOR_EACH (s, node, &remote->sessions) {
+        s->read_only = read_only;
+    }
+}
+
 /* Sets the options for all of the JSON-RPC sessions managed by 'remote' to
  * 'options'.
  *